Inhaled Nitric Oxide (iNO) and Chronic Lung Disease (CLD) Crossover Trial: At The Children's Hospital of Philadelphia and the Hospital of the University of Pennsylvania, we are currently conducting a double blinded, randomized, crossover clinical trial to examine the short term efficacy and safety of low dose iNO (10ppm) vs. standard oxygen therapy in preterm infants expected to develop CLD. Infants are eligible for enrollment if they are >21 days of age and ventilated with a mean airway pressure x FiO2 > 2.5. Infants are randomized to one of two treatment assignments: iNO (10ppm) for 4 days followed by placebo (nitric oxide analyzer and tank in place, set at 0 ppm) or placebo for the first 4 days followed by iNO. At randomization, envelopes, prepared by the Division of Biostatistics at the Children's Hospital of Philadelphia, are opened by the respiratory therapist, and only the the therapist is aware of treatment assignments. Physicians, nurses, and parents remain blinded. The primary outcome of this study is the change in FiO2 following a 4 day course of iNO, compared to the change in FiO2 following 4 days of placebo. At the end of both treatments, infants who have a > 20% decrease in FiO2 within one of the treatment periods receive a prolonged course of the more effective treatment. Blinding is maintained with weaning of iNO by 2ppm (or sham weans) every 4 days. In the first 6 months, 8 infants have completed trials of iNO. All of the parents approached have consented to their child's enrollment in the study. Blinding to clinicians and parents has been maintained by covering iNO analyzers, flowmeters, and pressure gauges. Treatment assignments remain blinded; however, 6 of 8 infants had a > 20% reduction in FiO2 within one of the treatment periods. They received a prolonged course of treatment with a study gas, with continued improvement in oxygenation. No adverse events (methemoglobinemia, bleeding, or progression of intraventricular hemorrhage) have occurred in study infants.